Regulation of mesocortical and striatonigral pathways by drugs aimed at inhibiting or enhancing dopaminergic neurotransmission is a commonly used strategy for treating many psychiatric and neurodegenerative diseases. These drugs act through dopamine receptors to elicit both short and long term changes in ion channel activity, protein kinase activities, and gene expression. The recent discovery of multiple G-protein-linked dopamine receptor subtypes exhibiting either classical "D1"-like or "D2-like" ligand binding and signal transduction properties raises several questions regarding the specific functions served by each receptor. Currently there are at least two D1-like and at least three D2-like DA receptors known. My previous immunohistochemical analyses of several D1- and D2-like subtypes showed that each receptor protein has a unique cellular and subcellular distribution within mesocortical, mesolimbic, and nigrastriatal pathways. These results support the notion that each subtype serves a unique function. However, subtype-specific signal transduction differences have not yet been identified in vivo. Without this functional information, it is difficult to understand the physiologic requirements for multiple D1-like and D2-like receptor subtypes. The goal of my research is to identify molecular and cellular processes that differentially regulate DA receptor function and localization in vivo using the D1-like receptors as a paradigm. One experimental approach involves use of the yeast two-hybrid system to identify proteins that potentially specify subtype-specific coupling to signal transduction systems or subcellular localization. My strategy is to use unique sequences within the closely related hD1 and hD5 subtypes as bait for detecting interacting proteins expressed in human brain. In complementary approaches, I will use subtype-specific monoclonal antibodies and affinity chromatography to isolate interacting proteins. Subtype-specific antibodies will also be used to confirm receptor interaction with proteins identified in the two-hybrid screen through immunoprecipitation experiments. This research will lead to a better understanding of the molecular organization of the DA receptor system, and identify new molecular targets for treating psychiatric and movement disorders.